Method and plant for washing of cellulosic pulp under overpressure



Sept. 12, 1961 J. c. F. c. RICHTER EI'AI. 2,999,785 METHOD AND PLANT FOR WASHING OF CELLULOSIC PULP UNDER OVERPRESSURE Filed July 17, 1959 2 Sheets-Sheet 1 INVENTORS c/OHH/V 67/. C'. Fla/Ins BY K ATTORNEYS p 1961 J. c. F. c. RICHTER ET AL 2,999,785

METHOD AND PLANT FOR WASHING OF CELLULOSIC PULP UNDER OVERPRESSURE Filed July 17, 1959 2 h ts- 2 FIG. 2.

INVENTORS r/o/m/v C'Fa Flaw-1? HALF (/Ohfl/VSF/V BY 5% 4 b? ATTORNEYS 2,999,785 Patented fiept. 12, 1961 2 999 7 85 METHOD AND PLANT FdR WASHING F CELLU- I LOSIC PULP UNDER OVERPRESSURE Johan Christofier Fredrik 'Carl Richter, St. Jean Cap Fen-at, Alpes Maritimes, France, and Rolf Jomar o- 5 hansen, Karlstad, Sweden, assignors to Aktiebolaget Kamyr, Karlstad, Sweden, :1 Swedish company Filed July 17, 1959, Ser. No. 827,954 4 Claims. (Cl. 162-60) The present invention relates to a method of washing tank. Particularly, the invention relates to the separation of lye from pulp, which after having been digested at high pressure and high temperature in a digester is transferred to a blow tank held under a reduced overpressure wherein the pulp still has a temperature .of above 100 C., e.g. 105 to 120 C.

The object of the invention is to gain advantages with regard to the heat economy by performing the washing at overpressure in a closed filter and by using blow steam for the maintenance of the pressure. Therefore, the invention is essentially characterized in that the pulp while still being under overpressure is carried to the pulp vat of the filter, and in that steam from the blow tank is supplied separately to a casing enclosing the sieve drum so that the part of the outside of the sieve drum located above the pulp level of the filter is held surrounded by steam under overpressure, whereas the interior of the drum is held essentially at atmospheric pressure, blow steam being pressed through the pulp layer upon the filter drum and dewatering the same. The use of steam means is an advantage above the common method of forcing air through the pulp, because air has an unfavourable influence upon the cellulosic material at the high temperatures used. In case the pulp in the blow tank is held at a temperature of above 100 C., e.g. 105 to 120 C., it is suitable in connection with the dilution of the pulp to a density suitable for the filter operation and for its transfer to the filter, to reduce the temperature of the pulp from the value present in the blow tank to a value of 100 to 110 C. On the other hand, the pressure of the pulp is preferably maintained; however, it may be reduced by control valves or similar but not more than to an overpressure sufliciently for the operation of the filter. The washwater supplied to the spray tubes of the filter is preferably of a temperature of above 100 C., e.g. 105 to 110 C. If the temperature of the spray water is chosen comparatively high, the same gives off steam when ejected and thus covers part of the steam required.

The invention also relates to a device for carrying out the above-mentioned method, which device comprises a wash filter having a rotary sieve drum arranged in a closed pressure-resistant casing. In addition to the pulp inlet said casing is provided with a separate steam inlet connected to the blow tank, and the interior of the filter drum is connected to a steam condenser. Preferably also a pressure air inlet is connected to the casing in order to make it possible when starting the filter and for reserve purposes to use pressure air instead of steam for creating a pressure difference between the outer and inner side of the filter drum. As it cannot be completely avoided that air and other non-condensible gases accompany the pulp, it is suitable to connect a fan to the condenser in order to discharge such gases. Said fan gives a maximum pressure of merely a few inches water column, and no pump or compressor for maintaining the comparatively high pressure difference between opposite sides of the filter surface is required.

The invention will be more closely described hereinbelow with reference to the accompanying drawings showing a plant adapted for carrying out the new washing method. FIG. 1 is a diagrammatic view of said plant in its entirety, the filter being shown in crosssection, and FIG. 2 is a partially sectional view at a greater scale of the filter and some details connected thereto.

In FIG. 1, the numeral 11 designates a blow tank which is periodically connected to the bottom outlet of a cellulose digester (not shown), in which wood chips, straw or similar fibre material has been digested at a high pressure and a high temperature by digestion with a suitable digesting liquor which may be sodium lye. When blowing the digester the digested cellulosic pulp together with used digesting lye flows through the conduit 13 into the blow tank 11, the pulp being defibrated due to the instantaneous pressure drop, and steam and usually also other gases are given off, which are let out through the conduit 15. Inserted into said conduit is a pressure controlling valve 17 by which a desired overpressure and a corresponding temperature is maintained in the blow tank. Preferably an overpressure of a few atmospheres at the most and a temperature of 105 to 120 C. are maintained. The pulp is discharged continuously from the lower end of the blow tank, the same thus also serving as an equalization container.

. Preferably the .blow tank is common to a number of batch digesters being blown successively. The blow tank can also be used for receiving pulp from a continuously operating digester, in which case its volume can be considerably reduced.

For the separation of the digesting lye from the pulp a wash filter is used designated in its entirety by the numeral 19 and essentially comprising a rotary cylindrical sieve drum 21 and a pressure-resistant casing 23 therefor, the lower part of which serves as a pulp vat. At its opposite ends and near the circumference the sieve drum is provided with cylindrical flanges 2.5, 27 which are sealed to corresponding cylindrical flanges 29, 31 of the casing by means of rubber straps G3 bridging the gap.

A roller 35 for picking olf the pulp layer bears against a straight line on the sieve drum, and a sealing rubber flap 37 is held against the periphery of the sieve drum further down. The members 35, 37 form a seal between the casing and the surface of the sieve drum and surround a rectangular opening 39 through which the pulp layer 41 formed upon the outside of the sieve drum is carried out of the pressure chamber without simultaneously letting out pressure fluid. One end plate of the sieve drum has an opening 43 through which the interior of the sieve drum communicates with a closed outlet chamber 45 arranged atone end of the filter. Arranged at the pulp outlet opening 39 and below the roller 35 is a trough 47 extending in the axial direction of the filter and having a conveyer screw 49 carrying oil? the pulp. Preferably, the walls of the trough 47 are joined to the outside of the casing 23, thus forming a chamber 51 separated from the atmosphere.

The pulp is transferred to the above-described filter in the following manner: At the lower end of the blow tank 11 the pulp is diluted with dilution water supplied through the spray nozzles 53 while being stirred by a motor-driven stirring device 55, and is carried therefrom through the conduit 57 in which a pump 59 can be inserted, to the pulp inlet 61 of the filter, which inlet is connected to the lower part of the filter casing 23 serving as pulp vat. A level feeling means 63 actuates a control device 65 comprising a valve 67 inserted in the conduit 57, thereby maintaining the pulp at such a level that the sieve drum Z1 dips into the pulp with merely 0 a part of its circumference.

Arranged at the top of the filter casing 23 is an inlet or to the conduit 71 through which fresh steam is supplied.

tFor reserve purposes and for the start, pressure air instead of steam may be required for maintaining the pressure of the filter and therefore the conduit 69 can also be connected to a pressure-air conduit 73. The pulp supplied to the filter also gives oil or is accompanied by a certain quantity of steam, particularly when the pressure of the pulp is reduced during the transfer from the blow tank, but said steam quantity is not sufficient for maintaining the desired pressure of the filter. Preferably, a pressure is maintained in the filter which is merely slightly lower than the pressure of the blow tank. The temperature of the pulp is lowered due to its dilution, and the pulp arriving to the filter is of a temperature of approxirrnately 100 to 110 C. In the filter the pulp is dewatered in the usual manner and the pulp layer 41 upon the sieve drum is carried below the spray tubes '75 through which washwater is supplied. Said washwater preferably is held at a somewhat higher temperature than the pulp, cg. at 105 to 110 C., whereby it gives oh" steam when ejected and thus reduces the requirement of steam to be supplied from conduit 69. Steam of overpressure flows through the pulp layer upon the sieve drum into the interior of the sieve drum, thus displacing and carrying with it digesting lye and washwater, so that the pulp layer is dewatered and washed. Connected to the outlet chamber 45 of the filter is a vertical, downwardly extending, comparatively Wide tube 77 through which the lye separated from the pulp entering the filter drum and the spray water are carried oil. From the lower end of said vertical tube the filtrate is pumped by means of a pump 79 into a conduit '81 to which the conduits 33 and 85 are connected. The conduit 83 carries the main part of the filtrate back to the spray nozzles 53 of the blow tank, and the conduit 85 carries the rest of the filtrate to a plant for recovery of the chemicals therein. Inserted in the latter conduit is a valve 87 controlled by a level feeling means 89 in such a manner that the tube 77 is held almost filled with filtrate. The water column of the tube 77 having a length of several feet causes a hydrostatic pressure preventing steam from being formed in the pump '79. By recirculating the filtrate to the outlet end of the blow tank the pulp is diluted to a consistency suitable for the operation of the filter, e.g. to a dry content of a few percent. By controlling the quantity of filtrate circulating between the filter and the blow tank and also by controlling the temperature of the water supplied to the spray tubes 75 of the filter it is easy to arrange that the temperature drop of the pulp in connection with its transfer to the filter assumes the desired above stated value.

Connected to the top of the outlet chamber 45 of the filter is a conduit 91 leading to a condenser 93. The interior of said condenser communicates with the atmosphere through a conduit 95, in which a fan 97 is inserted. Cooling water is supplied to the condenser through a pipe 99. Condensate and used cooling water flows through a conduit 101 to a collecting tank 103. On account of the interior of the sieve drum being connected to a condenser as described, the steam passing through gases accompanying the blow steam, which gases can- 4 not be condensed in the condenser and therefore would increase the pressure therein if they were not discharged.

When a closed pulp outlet chamber 51 is provided, the same is also put in communication with the condenser 93 by means of a conduit 105, so that the steam is carried oil which is liberated from the pulp due to the pressure drop at the roller 35.

When a closed pulp outlet chamber 51 is provided, the same is also put in communication with the condenser 93 by means of a conduit 105, so that the steam is carried oil which is liberated from the pulp due to the pressure drop at the roller 35.

The wash water supplied to the spray tubes 75 of the filter is preferably taken from the collecting tank 103 and pumped by a pump 107 through a heat exchanger 109 in which it is heated by steam taken from conduit 71, to the desired above-mentioned temperature. Said temperature is automatically maintained by means of a control device 111 influencing the steam supply. The steam condensed in the heat exchanger 109 flows through the conduit 113 to the tank 103.

The above-described method and device can be used also in the case that the pulp that should be washed consists of pulp taken from a treatmentat overpressure with any other chemicals than those used for digestion, e.g. alkalic treatments of previously digested pulp.

What is claimed is:

1. Apparatus of the character described comprising a blow tank; means for introducing, into said tank, cellulosic pulp blown from a digester under superatmospheric pressure; an outlet at the top of said blow tank for letting out steam released from the pulp in said tank at a reduced, but still superatmospheric, pressure; a pulp outlet at the bottom of said blow tank; a wash filter defined by a rotary sieve drum and a closed pressureresistant casing surrounding said drum; conduit means connecting said pulp outlet to said filter casing for introducing pulp at said reduced superatmospheric pressure into the space between said casing and said sieve drum; an inlet in said casing above the pulp level therein; conduit means connecting said steam outlet from said blow tank to said inlet, for applying said reduced superatmospheric steam pressure to the exterior of said sieve drum; and a steam condenser in communication with the interior of the drum for establishing a still more reduced pressure within said drum.

2. A method of washing digesting liquor fi'orn cellulosic pulp supplied from a blow tank to a filter having a rotary sieve drum, by means of steam liberated from said pulp by dropping the pressure thereon at the introduction of the pulp into said blow tank, which method comprises carrying the pulp while still under superatmospheric pressure from the blow tank to the pulp vat of the filter; forming a layer of pulp on at least the upper exterior surface of said sieve drum;.separately introducing said steam from said blow tank into a casing enclosing said sieve drum, and above the liquid level of said pulp vat therein; thereby maintaining the part of the exterior of said sieve drumlocated above the pulp level in said filter surrounded by steam of said superatmospheric pressure; maintaining at the same time, the interior of said sieve drum at essentially atmospheric pressure; and continuously passing said blow steam through said pulp layer and into the interior of said drum, to displace or carry with it part of the liquid content of said ul p 3 A method as claimed in claim 2, in which the pulp of blow tank is maintained at a temperature of above 'C., e.g. to 120 C., characterizedin that in connection with the dilution of the pulp to a consistency suitable for the filter operation and for the transfer of the pulp to the filter, the temperature of the pulp is lowered from the value present in the blow tank to a value of 100 to C.

.4. A method as claimed in claim 2, charactenzcd m 6 that washwater heated to a temperature of about 100 1,762,560 Morton ...1... June 10, 1930 C., preferably to 105 to 110 C., is sprayed upon the 1,871,207 Whitman Aug. 9, 1932 pulp layer formed on the sieve drum. 2,500,056 Barr Mar. 7, 1950 2,711,359 Johansen June 21, 1955 References Clted 1n the file of th1s patent 5 2,741,369 Fest "v Apr. 10, 1956 UNITED STATES PATENTS 2,899,068 King Aug. 11, 1959 1,512,321 Wait Oct. 21, 1924 

2. A METHOD OF WASHING DIGESTING LIQUOR FROM CELLULOSIC PULP SUPPLIED FROM A BLOW TANK TO FILTER HAVING A ROTARY SIEVE DRUM, BY MEANS OF STEAM LIBERATED FROM SAID PULP BY DROPPING THE PRESSURE THEREON AT THE INTRODUCTION OF THE PULP INTO SAID BLOW TANK, WHICH METHOD COMPRISES CARRYING THE PULP WHILE STILL UNDER SUPERATMOSPHERIC PRESSURE FROM THE BLOW TANK TO THE PULP VAT OF THE FILTER, FORMING A LAYER OF PULP ON AT LEAST THE UPPER EXTERIOR SURFACE OF SAID SIEVE DRUM; SEPARATELY INTRODUCING SAID STEAM FROM SAID BLOW TANK INTO A CASING ENCLOSING SAID SIEVE DRUM, AND ABOVE THE LIQUID LEVEL OF SAID PULP VAT THEREIN; THEREBY MAINTAINING THE PART OF THE EXTERIOR OF SAID SIEVE DRUM LOCATED ABOVE THE PULP 